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of facts distinguished by Darwinian writers concern the 

 history and relations of these phenomena of structure. 

 Palaeontology gives us evidence concerning the history of 

 structure in past epochs of time, while variation in nature 

 and under artificial conditions shows us changes taking place 

 at the present time. 



Accepting the conclusion that all changes of structure 

 have occurred in the course of the natural succession of 

 generations, the problem before students of animal evolution 

 at the present time is to discover the causes which have 

 produced these three kinds of structural difference in 

 animals. 



We may consider first the peculiarities which distinguish 

 animals of different and separate lines of descent, or pedigrees. 

 These are conveniently termed diagnostic characters, and 

 include all those by which the groups of various degrees are 

 distinguished for the purpose of classification. Thus we have 

 specific characters, generic characters, ordinal characters, and 

 so on to the higher and more general subdivisions called 

 sub-kingdoms or phyla. 



Animal structure is not, like the shapes of crystals, entirely 

 a matter of abstract form. Animals are living creatures, 

 and life depends upon relations between the organism and 

 the surrounding world. 



The essentials of animal existence are the acquisition 

 of food, the escape from enemies, and the generation of 

 offspring. Under different conditions these objects are 

 attained in different ways. In the most familiar animals we 

 observe in the bodily structure striking and complicated 

 mechanisms for attaining them. Such co-ordinated structural 

 adjustments are called adaptations. The study of the modes 

 in which the structure of the bodies of animals enables them 

 to maintain their existence leads to two great questions : 

 (1) Is everything in structure essential or advantageous to 



